A new gene therapy approach capable of delivering full-length versions of large genes and improving skeletal muscle function may hold new hope for treating dysferlinopathies and other muscular dystrophies, scientists say.

The challenge of treating patients with genetic disorders in which a single mutated gene is simply too large to be replaced using traditional gene therapy techniques may soon be a thing of the past.

A group of untreatable muscle disorders called dysferlinopathies are caused by mutations in the dysferlin gene. The patients with these disorders, including limb girdle muscular dystrophy type 2B, are typically diagnosed during their early twenties. Approximately one-third of the sufferers will become wheelchair dependent by their mid-30s.

Gene therapy using adeno-associated virus (AAV) to deliver genes to cells has been taken up as an option for some patients with muscular dystrophy.

But, AAV”s packaging limitations have served as obstacles in using gene therapy to deliver large genes like dysferlin.

“We have had success in the clinic using AAV gene therapy with limb girdle muscular dystrophy type 2D, which is caused by mutations in the alpha-sarcoglycan gene,” Louise Rodino-Klapac, PhD, principal investigator in the Center for Gene Therapy at The Research Institute of Nationwide Children”s Hospital, said.

“However, the dysferlin gene is very large, about six times larger than the alpha-sarcoglycan gene and can”t fit into a traditional AAV vector.”

However, a 2008 study identified AAV5, an AAV serotype that could package large transcripts.

“This made us wonder whether it could be used for gene replacement requiring inserts as large as the dysferlin gene,” Dr. Rodino-Klapac said.

In their 2012 study, Dr. Rodino-Klapac”s team used AAV5 to package a full-length, intact dysferlin gene and directly deliver it to the diaphragm of dysferlin-deficient mice.

They also injected the leg muscles of dysferlin-deficient mice using both intramuscular and vascular approaches to further calculate whether the gene delivery could improve skeletal muscle function.

They discovered that both the intravascular and intramuscular delivery approaches led to full-length, intact dysferlin gene expression in the leg and diaphragm muscle cells of the mice.

More importantly, they found that the newly-restored dysferlin repaired membrane deficits previously seen in the dysferlin-deficient mice.

“With regard to neuromuscular diseases, these studies provide new perspective for conditions caused by mutations of large genes. Duchenne muscular dystrophy is the most common severe childhood muscular dystrophy and would seem to benefit from expression of the larger transcripts than mini- and micro-dystrophins that only partially restore physiologic function in mouse models of the disease,” she said.

“We have shown that AAV5-dysferlin delivery is a very promising therapeutic approach that could restore functional deficits in dysferlinopathy patients,” she added.